Magnetic deflecting means for cathode ray tubes



Nov. 29, 1955 E; w. BULL 2,725,496

MAGNETIC DEFLECTiNG MEANS FOR CATHODE RAY TUBES Filed Nov. 10, 1952 2 SheetsSheet l lm ea 3/ ERIC WILL/AM BULL 6 651%??? E. W. BULL Nov. 29, 1955 MAGNETIC DEFLECTING MEANS FOR CATHODE RAY TUBES 2 Sheets-Sheet 2 Filed Nov. 10, 1952 FIGS.

United States Patent MAGNETIC DEFLECTING MEANS FOR CATHODE RAY TUBES Eric William Bull, Honnslow, England, assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application November 10, 1952, Serial No. 319,712

Claims priority, application Great Britain November 24, 1951 7 Claims. (Cl. 313-77) This invention relates to magnetic deflecting means for a cathode ray tube and is particularly concerned with means of this kind comprising a deflecting coil having its turns wound round a magnetic yoke that is arranged to define an air space whhich is adapted to contain that portion of the cathode ray tube at which deflection of the cathode ray beam occurs, said deflecting means being adapted when energised to set up a magnetic flux within said air space.

One known means of this kind comprises a magnetic yoke which is arranged to surround the neck of the cathode ray tube and deflecting coils the turns of which are arranged round oppositely disposed portions of the yoke such that when the coils are energised a magnetic field is set up within the air space defined by the inner boundary of the magnetic yoke. Magnetic deflecting means of this kind are described in the specification of British Pat out No. 612,906.

A diificulty experienced hitherto with this kind of deflecting means is that only a proportion of the total magnetic flux set up by the coils passes through the air space defined by the magnetic yoke and does useful work in deflecting the cathode ray beam. The remainder of the magnetic flux passes outside this space and, in addition to being so much waste flux, it tends to create electrical disturbances in other parts of the apparatus associated with the cathode ray tube. Furthermore, the inductance of the deflecting coils becomes relatively high due to this waste flux and this tends to cause excessive voltages to be induced in the coils when the magnetic field collapses during retrace periods of the cathode ray beam.

An object of the invention is to reduce or substantially eliminate the waste flux normally set up by magnetic deflecting means of the kind specified.

Another object of the invention is to prevent excessive voltages from being set up in the circuit of magnetic deflecting means of the kind specified during retrace periods of the scanning beam.

According to the present invention there is provided magnetic deflecting means for a cathode ray tube of the kind comprising a magnetic yoke adapted to surround the neck of said tube and a deflecting coil or coils on said yoke which when suitably energised set up a magnetic deflecting field within said neck and a further field outside said yoke and having a further coil or coils so disposed that when suitably energised serve to neutralise, said further field to a greater extent than said deflecting field.

Usually two compensating coils will be used to obtain maximum compensation and these may be arranged on the yoke beyond the ends of the deflecting coil or coils. When the magnetic yoke is provided with oppositely disposed sections or limbs and with oppositely disposed de fiecting coils, each surrounding one section or limb, the compensating coils may also be disposed symmetrically on the yoke so that each encircles both deflecting coils. In either case the magnetic yoke provides a path of low reluctance for the compensating magnetic field set up by the compensating coils and forms an effective screen be- Patented Nov. 29, 1955 2 tween the compensating field and the cathode ray tube so that the direct effect of the compensating field upon the cathode ray beam is small.

According to a feature of the invention, the compensating means may be arranged to prevent excessive voltages from being set up in the circuit of the deflecting means when the magnetic field set up by the latter collapses during retrace periods of the cathode ray beam. One method consists in connecting the compensating means in opposed series relation to the deflecting coil or coils and coupling them together with a suitably high coupling factor so as to obtain a relatively high value of mutual inductance in the circuit. Thus, to take a simple example, if a single deflecting coil of inducance value L1 and a single compensating coil of inductance value L2 are connected together in series so as to have a mutual inductance M, the equivalent or overall inductance L of the circuit is given by the formula In the present case, since the coils must be arranged so that their magnetic fields are in opposition the appropriate formula is:

It will be apparent that the value of the overall inductance L will be less than the inductance L1 of the deflecting coil if 2M is greater than L2 and this result will be achieved if the coupling factor is given a suitable high value. Since the voltage induced in the circuit is proportional to the inductance of the circuit it follows that the induced voltage will be reduced when a suitable compensating coil is arranged in a manner to reduce the overall inductance of the circuit.

In order that the said invention may be clearly understood and readily carried into effect, a number of embodiments will now be described with reference to the accompanying drawings, in which:

Figure 1 shows diagrammatically a front view of a portion of a magnetic yoke carrying a deflecting coil and provided with compensating means according to the invention,

Figures 2, 3 and 5 show diagrammatically front views of a number of closed rectangular magnetic yokes provided with oppositely. disposed deflecting coils and different arrangements for compensating the coils in accordance with the invention, A

Figure 4 is a view fronrthe shown in Figure 3,

Figure 6 shows, in perspective, a practical embodiment of a deflecting coil unit suitable for the cathode ray tube of a television receiver,

Figure 7 shows the unit of Figure 6 fitted with compen sating coils according to the invention,

Fignreg8 shows the unit of Figure 6 fitted with compensating coils of a. modified form, and

Figure 9 shows the compensated unit of Figure 8 surrounded by a magnetic screen.

Referring to Figure l, a portion of a magnetic yoke 10 is shown comprising a limb 11 and oppositely disposed limbs 12 and 13, the lower portions of which are cutaway in the drawing. The yoke 10 defines an air space A for a cathode ray tube (not shown) which is adapted to contain that portion of the tube at, which defl'ection of the cathode ray beam occurs. A deflecting coil 114 is wound round limb 11- and, when energised by oscillatory current, the coil sets up a magnetic deflecting field which includes a useful flux F1 which traverses the air space A and serves to deflect the cathode ray beam, and a waste or leakage flux F2 which lies outside the space A. Elements of the fluxes F1 and F2 are indicated, respectively, by-t-he chain "lines 16 and 16.

top of the arrangement Tn order to reduce or substantially neutralise the waste flux F2, a conductor 15, seen in end view in Figure 1, is disposed above coil 14 so that it extends from front to rear of the yoke with its aXis normal to the axis of coil 14. Conductor 15 may be suitably energised such that it sets up a compensating magnetic field F3 in opposition to the magnetic deflecting field set up by coil 14. An element of the compensating flux F3 is indicated by the chain line 17 and this element opposes and substantially neutralises element 16 of the waste fluX F2 so that only the uscr-ul element 16 of the deflecting flux shown in Figure 1 remains. It will be apparent that by suitably disposing one or a number of conductors 15 outside yoke and passing current of suitable value and frequency through it or them a compensating magnetic field F3 may be set up which opposes the deflecting magnetic field and neutralises the waste component F2 of that field to a greater extent than the useful component F1 thereof. l loreover, since the limb 11 of yoke 10 forms a path of low reluctance for the compensating flux F3 it effectively screens the air space A from flux F3 and that flux has little direct effect upon the cathode ray beam.

In order to reduce the value of the current required to set up the compensating field it is preferable to replace conductor by a group of conductors connected in series. Conveniently this may be accomplished by winding an insulated conductor to form a compensating coil and an arrangement of this kind is shown diagrammatically in Figure 2. A closed rectangular magnetic yoke 18 is formed with oppositely disposed pairs of limbs or sections w, 2 and 21, 22 and with oppositely disposed deflecting coils Z3 and 24 of which coil 23 is arranged round limb l9 and coil 24 round limb of one of said pairs of limbs. As shown, the coils 23 and 24 are formed in two sections. The sections of each coil are connected in series and the two coils are connected in parallel. Coils 23 and 24 may, however, be connected in series if desired.

A pair of compensating coils 25, 28 are symmetrically disposed on the yoke 18 beyond the ends of the deflecting coils 23 and 24 so that each coil embraces both limbs 19 and 20 of the yoke 18. As shown, the coils and 28 are arranged to surround limbs 22 and 21, respectively, of the yoke so that they do not obstruct the air space A. Compensating coils 25 and 28 are shown connected in series in the deflecting coil circuit and the arrangement is such that, when that circuit is energised, they set up magnetic fields which oppose the deflecting magnetic fields set up by the deflecting coils 23, 24. The

active sections of compensating coils 25, 28 which set up the compensating magnetic fields are the conductors 26, 26' at the top of the yoke and conductors 27, 27 at the bottom. The intermediate sections of coils 25 and 28 which connect the active sections 26, 27 and 26, 27 of the coils together may be regarded as idle sections since the compensating effect of the magnetic field set up round them is of minor importance. These idle sections are arranged to lie close to the limbs 22, 21 of yoke 13 so that the paths of low reluctance provided by these limbs largely confine the unwanted fluxes set up by these sections and tend to prevent them from extending into the path of the cathode ray beam. The compensating coils may, of course, be suitably connected in parallel if desired and in parallel with the deflecting coils. The arrangement shown in Figure 2 provides a close coupling between the deflecting coils and the compensating coils and a high value of mutual inductance such that the overall inductance of the compensated deflecting means is less than the inductance of said means when the compensating coil or coils are removed.

Figures 3 and 4 show two views of deflecting means similar to the deflecting means illustrated in Figure 2 but provided with a modified type of compensating coil. In this case the compensating coils 30 and 31 are shaped like the well known saddle type of deflecting coil and have active sections 32, 33, 34 and 35 surrounding defleetin coils 23 and 24- which set up compensating magnetic fields. These active sections are symmetrically disposed adjacent to the vertical centre line of the yoke 10 and the idle sections 36, 37, 38 and 39 of the compensating coils which project forwardly and rearwardly of yoke are displaced relative to the active sections 32, 33, Id and 35, by turning them outwardly so that they follow the contour of yoke 18 and do not obstruct entry to air space A.

r 'gure 5 shows magnetic deflecting means similar to that shown in Figure 3 but fitted with diflerently shaped saddle type compensating coils 4-1, 42. In this case the sections of the coils 41, 42 are turned down at i3 and and up at 44 and 46 and then turned outwardly, such that these idle sections follow the centre lines of limbs 19, 2e, 21, 22 of yoke 18. The performance of the arrangement shown in Figure 5 has been found to be slightly better than that of the arrangement shown in Figure 3 and also better than a very similar arrangement in which the idle sections of the compensating coils are arranged to follow the internal contour of the tagnetic yoke 18.

Figure 6 shows a practical embodiment of a deflecting coil unit used for the cathode ray tube of a television receiver. It comprises a square magnetic yoke having limbs 5b, 51, 52 and 53 formed of a material sold under the Registered Trade Mark Ferroxcube which has a high permeability and low hysteresis and eddy current losses television line scanning frequencies of the order of 10,000 cycles per second to 100,000 cycles per second. Coil formers 54 mounted on limbs 50, 51, 52 and 53 carry a pair of line deflecting coils 55, 56 and a pair of frame deflecting coils 57, 58, each coil being made up of two sections connected in series. Figure 7 shows the unit of Figure 6 fitted with a pair of compensating coils 59, 60 which compensate for the waste flux of the magnetic deflecting field set up by the line deflecting coils and 5-5. The coils 55 and 56 are arranged in the manner described with reference to Figure 2. Figure 8 shows the same unit fitted with saddle type compensating coils 62 and 63 of the kind described with reference to Figure 5. Although the frame scanning coils of this unit set up a deflecting magnetic field which includes a proportion of waste flux, the effects of this flux can usually be tolerated. If desired, however, additional compensating coils may be mounted on the unit and suitably disposed so as to neutralise the waste flux set up by the frame scanning coils 57 and 58.

Figure 9 shows the compensated magnetic deflecting means of Figure 8 surrounded by a two'part magnetic screen, made up of the members 64, 65, which shields the deflecting coils from stray external magnetic fields. The use of a magnetic screen without compensating means would normally increase the ratio of waste to useful flux of the line deflecting magnetic field and also introduce hysteresis and eddy current losses due to the waste flux of the line deflecting field entering that member. When, however, this waste flux is neutralised by the use of compensating means according to the invention, the ratio remains substantially unchanged and the losses introduced are negligible.

Preferably the compensating coils are wound with the same gauge or a thicker gauge wire than the gauge of wire used for the deflecting coils so as to avoid introducing disproportionate copper losses.

It will be understood that the deflecting means illustrated in Figures 6 to 9 are each adapted to be mounted on a cathode ray tube so that the magnetic yoke surrounds the neck of the tube; that the line and frame deflecting coils will be suitably connected in scanning circuits fed with oscillatory currents, usually of sawtooth waveform, such that the line scanning coils and frame scanning coils are adapted to cause, respectively, horizontal and vertical deflection of the cathode ray beam; and that the compensating means described with reference to Figures 7 to idle 9 will be suitably connected in the circuit of the line scanning coils.

By applying the invention to magnetic deflecting means of the kind illustrated in Figure 6 it has been found possible to obtain an overall reduction in the inductance of the line deflecting coils of over 40 per cent with a reduction of sensitivity of only per cent. These figures show that the effect of the compensating flux is to neutralise a substantial proportion of the waste flux set up by the line deflecting coils but only a small proportion of the useful flux which causes line deflection of the cathode ray beam.

Although the invention has been described with particular reference to magnetic deflecting means provided with closed rectangular magnetic yokes, it will readily be understood that it may also be applied when other forms of yokes are used such as, for example, an open magnetic yoke provided with pole pieces which define an air space for the cathode ray tube or a closed circular magnetic yoke.

What I claim is:

1. Magnetic deflecting means for a cathode ray tube, comprising a magnetic yoke defining an air space for the neck of a cathode ray tube, deflecting coil means wound on to said yoke to set up a deflecting magnetic field in said air space and a stray magnetic field outside said yoke, and further coil means connected in series with said deflecting coil means disposed outside said yoke to set up an opposing magnetic flux screened from said air space by said yoke and lying substantially wholly outside said air space and neutralizing said stray magnetic field to a greater extent than said deflecting magnetic field.

2. Magnetic deflecting means for a cathode ray tube, comprising a rectangular magnetic yoke provided with two pairs of parallel limbs forming a closed magnetic circuit adapted to surround the neck of the cathode ray tube, a pair of oppositely disposed deflecting coils, one surrounding one limb of a parallel pair and the other surrounding the other limb of said parallel pair and setting up a magnetic deflecting field within said neck and a stray mag netic field outside said yoke, and a further pair of coils setting up an opposing magnetic flux, said further coils being disposed outside said yoke one at each end of said parallel pair of limbs and encircling said other pair of parallel limbs.

3. Magnetic deflecting means for a cathode ray tube,

comprising a rectangular magnetic yoke provided with two pairs of parallel limbs forming a closed magnetic circuit adapted to surround the neck of the cathode ray tube, a pair of oppositely disposed deflecting coils one surrounding one limb of a parallel pair and the other surrounding the other limb of said parallel pair and setting up a magnetic deflecting field Within said neck and a stray magnetic field outside said yoke, and a further pair of coils setting upan opposing magnetic flux, said further coils being disposed outside said yoke and encircling said oppositely disposed deflecting coils.

Magnetic deflecting means according to claim 3, sections of said further coils projecting forwardly and rearwardly of said yoke and displaced relative to the remainder of said coils with said sections following the contour of said yoke.

5. Magnetic deflecting means for a cathode ray tube comprising a magnetic yoke providing a path for magnetic flux and defining an air space for the neck of a cathode ray tube, deflecting coil means comprising a pair of deflecting coils mounted on said yoke, each coil having turns encircling, respectively, oppositely disposed portions of said path with portions of said turns lying within said air space and setting up a deflecting magnetic field within said air space and other portions of said turns lying outside said yoke and setting up a stray magnetic field outside said yoke, and further coil means lying wholly outside said yoke and air space and neutralising said stray magnetic field to a greater extent than said deflecting magnetic field.

6. Magnetic deflecting means according to claim 5 wherein said further coil means comprise a pair of coils disposed one at each end of said pair of deflecting coils.

7. Magnetic deflecting means according to claim 5 wherein said further coil means encircle said pair of defleeting coils.

References Cited in the file of this patent UNITED STATES PATENTS 1,981,322 Nakajima et al. Nov. 20, 1934 1,995,376 Campbell Mar. 26, 1935 2,093,157 Nakashima et a1 Sept. 14, 1937 2,561,586 Montgomery July 24, 1951 2,569,343 Skull Sept. 25, 1951 2,619,612 Lawrence Nov. 25, 1952 

